Consider a regional power system having a strong internal transmission system transmitting power to another strong regional system on relatively weak Interties. Such a regional power system may experience issues with stability during disturbances, such as short circuits, loss of generation, loss of load, loss of one of the Interties, or any combination thereof. Prevalent practice to the solution of these issues is to include more Interties, increase the voltage to higher voltage levels (such as extra high voltage (EHV) levels or ultra high voltage (UHV) levels), or both. Another approach for better power system stability is to employ protection relays with high operation speed.
Travelling wave protection is one approach for super-high speed protection. There are different types of travelling wave protections, for example, travelling wave pilot protection based on directional comparison, travelling wave current differential protection, travelling wave protection based on distance measurements, etc.
In general terms, directional pilot protection enables a practical and reliable mechanism for travelling wave protection. It only needs a small bandwidth channel to transmit binary information between terminals at end points of a transmission line.
Travelling wave protection based on directional comparison has e.g., been presented in “Ultra High Speed Relay for EHV/UHV Transmission Lines'-Development, Design and Application” by M. Chamia and S. Liberman in IEEE Transactions on Power Apparatus and Systems, Vol. PAS-97, No. 6, November/December 1978. In general terms, in such protection system, the voltage and current at both ends of the protected line are measured. The direction of the local voltage and current measurements is detected. A trip decision is made based on a comparison of the directions of both ends.
In travelling wave protection based on directional comparison the polarities of the first wave fronts of local voltage and current are compared. If they are the same, a backward fault has occurred. If they are reverse, a forward fault has occurred. Then the protection relays at the two terminals will transmit the fault direction to the other terminal. If both directions are forward directions, an internal fault has occurred. Otherwise, an external fault has occurred. Its basic principle is shown in FIG. 1.
Some factors which may lead to reliability issues for known mechanism for travelling wave directional pilot protection are summarized next.
Firstly, directional pilot protection needs both voltage and current measurements on both terminals of the transmission line, which may have sensitivity issues when one of the voltage or current travelling wave is too small for polarity detection. For example, in an extreme strong system, the busbar voltage will not change during occurrence of the faults, which means the travelling wave Δu is always zero or close to zero. As a result, the directional pilot protection cannot detect the voltage polarity at the strong system side, and thereby the directional pilot protection will fail to operate.
Secondly, voltage compensation is a classical and practical way to resolve the sensitivity problem of pilot protection based on fundamental frequency values for extreme strong or weak systems. But unfortunately, this mechanism cannot be used for travelling wave protections, because the classical voltage compensation is based on fundamental frequency phasor and Ohm's law, which is not suitable for travelling wave protection based on high frequency fault transient. Some other mechanisms are needed to resolve the sensitivity problem for extreme strong or weak systems.
Hence, there is a need for improved protection of a transmission line, for example to enable improved travelling wave protection which can reduce the risk of failure of operation.